Interfacial reactions between Sn–9Zn + Cu lead-free solders and the Au substrate

The effects of Cu addition into Sn–9Zn solder reacting with an Au substrate at 160 °C for 24 h were investigated in this study. The intermetallic compound (IMC) evolution sequences at the Sn–9Zn + xCu/Au interface and in the solders were (i) the (Au3Zn7 + AuZn2 + AuZn) and Au3Zn7 phases at the Sn–Zn/Au couple; (ii) the (Au3Zn7 + AuZn), (Au, Cu)3Zn7 phases, as x was 1 wt%; (iii) the AuSn and Cu5Zn8 phases, as x was 4 wt%; (iv) ((Cu, Au)Sn + AuSn) and CuZn phases, as x was 7 wt% and (v) ((Cu, Au)Sn + AuSn) and CuZn phases at the Sn–9Zn + 10 wt%Cu/Au couple. When x is less than 3 wt%, the free Zn atoms in the liquid solder can easily react with the Au substrate to form Au–Zn IMC at the interface. The amount of active Zn atoms in the liquid solder decreases when 3–6 wt%Cu is added into the Sn–Zn solder. Thus, most of the Zn atoms react with Cu to form a Cu–Zn short-range-ordered structure IMC in the solder. Sn atoms then become the dominant diffusion specimens and react with the Au substrate to form the binary Au–Sn IMC at the solder/Au interface. When 6–10 wt%Cu is added into Sn–9Zn solders, the solder/Au interface converts completely into the Sn–Cu/Au system.